After completing DA: Awakening, I would conclude that Dragon Age is much more difficult game than BG2, if you compare each game on their hardest difficulty levels (Insane for BG2, Nightmare for Dragon Age).
The problem with BG2 is that there are way too many exploits to take advantage of. Hell, I wouldn't even call some of them exploits, just spells that turn what should have been hard battles into a cakewalk. A mage can gain virtual immunity to physical damage with low level spells (mirror image, stoneskin, protection from weapons), and AoE spells that are far superior in damage to Dragon Age (eg. Horrid Wilting). Then you have game breakers like Mislead, Time Stop, Project Image, Harm, and Wish.
I've mentioned this in another thread, but Dragon Age: Origins doesn't really have a 'silver bullet' that guarantees victory in more difficult fights. Mana Clash is an instant win against mages, but they are only 5% of the game's enemies in Origins, and enemy mages in Awakening have so much HP that they can survive a Mana Clash. Some people have argued that you just need to spam Cone of Cold, but that only works for the trash mobs (which, admittedly, is about 75% of the game). Against high ranked enemies, it's only going to disable them for a fraction of a second and deal mediocre damage due to HP bloat. With the high-end damage spells, either the lengthy cooldown limits your ability to spam them (eg. Hand of Winter), or the mana drain means you can only sustain them for a short period of time (eg. Elemental Chaos). What really makes things difficult is that your crowd control/disabling spells have their duration reduced by 80% against elite bosses, making them virtually useless.
If it is Baldur's Gate 1 vs. Dragon Age, then it's no contest. Baldur's Gate 1 is far more difficult. For a large portion of the game you can be slain after only being hit once. Your spell selection sucks. And the random encounters are *brutal*.